The invention relates to a hip joint socket for insertion into bone tissue and which has an outside metal shell and a ceramic shell anchored in the metal shell.
Hip joint endoprostheses consist of a hip joint socket which is anchored in the pelvic bone, and a ball which is rotatably inserted into the socket and is anchored to a shaft in the femur.
Hip joint sockets consist of an outer metal shell which constitutes the shape of the implant and includes an inner shell that is made of ceramic. The ceramic shell is mechanically anchored in the outer metal shell. There are also inner shells of plastic. The term, metal shell, is synonymous with the metallic part of the acetabular implant, whose outer shape can be configured in accordance with medical requirements.
It is state of the art to fix the inner ceramic shell in the metal shell by means of a taper lock. The angle of the taper lock is 5.degree. 43', i.e., an angle ratio of 1:10.
A disadvantage of this taper lock is that once inserted, a ceramic shell can no longer be removed from the metal shell because the seizing forces are too great. Thus it is difficult to replace the friction bearing component, i.e., the ceramic shell, during a surgical operation or in the case of a re-operation. Usually, the entire socket implant must then be removed or the ceramic inlay destroyed. This, however, leads to problems on account of the formation of splinters.
Another disadvantage of the taper lock is an undesirable load-beating function for the ceramic component. The latter is gripped by the outer taper ring and is then cantilevered on the domed side, resulting in considerable tensile stress on the component. In order to design a socket inlay to deal with the mechanical stresses considerable wall thicknesses (at least 4-5 mm) are necessary. As a result, the ceramic-metal sockets have a large outside diameter. However, from the medical viewpoint, small components are desirable in order to minimize bone loss.
The present invention is addressed to providing an unproved hip joint socket of the type previously described such that it will be of a small size with a thin wall thickness of the ceramic shell.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects obtained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention .